fix(LTX25-DEVICE-SEAM-SIBLING): the third question, and a leakage bucket that is a property rather than a spelling (#659, #660)

.agents/issue-index.md

@@ -224,6 +224,7 @@ rather than merged. `scripts/check-agent-record.py` gates both.
 | [#691](https://github.com/mudler/vllm.cpp/issues/691) | `ROAD-V1-LTX25` | The LTX-2.5 out-of-scope ledger's reachable/marker split gates the MESSAGE TEXT, not reachability: give `kLoraFusion` a product call site and the ledger test still passes while the shipped refusal tells the caller `DECLARED, NOT REQUESTABLE` right after they requested it. Copy the anti-tautological shape of the extras inventory gate | bug |
 | [#692](https://github.com/mudler/vllm.cpp/issues/692) | `ROAD-V1-LTX25` | Campaign spec `ltx-2-5.md:291` still records the FABRICATED `multishot` arm as owed after row `LTX25-RETIRE-DEAD-ARMS` retired it; same line needs `CFG parallelism` renamed to multi-GPU. Operator-owned file, so filed rather than edited by that row | bug |
 | [#886](https://github.com/mudler/vllm.cpp/issues/886) | `POLICY-NOTHING-LANDS-DEAD` | The seam checkers police where a capability routes and nothing polices whether anything reaches it, so a change can land wired downward and driven only by its own test, as tensor parallelism did (guide [`reachability.md`](reachability.md)) | feature |
+| [#828](https://github.com/mudler/vllm.cpp/issues/828) | — | `check-device-leakage.py`'s `dev_cast` bucket enforces a set of SPELLINGS, not the property "an integer becomes a `vt::DeviceType`": four review rounds each found a spelling the previous round's message already claimed, every one closing at ZERO hits, so widening is not the answer. The structural fix is an AST-level check (clang tooling), where the destination type canonicalises and the source type is known. Listed under `## Owed` in [`ltx25-device-seam-sibling.md`](specs/ltx25-device-seam-sibling.md) | feature |
 | [#673](https://github.com/mudler/vllm.cpp/issues/673) | — | LTX-2.5 prompt-AdaLN: the row's checkpoint-derived evidence is MANUAL and host-local. `LTX2_CHECKPOINT_ROOT` is set by no workflow (`grep -rn CHECKPOINT_ROOT .github/` exits 1 with zero hits, positive control matches in `tests/` and `.agents/`), so CI executes 784 of 9031 assertions — 8.7%, measured 2026-08-15 — of `test_ltx2_video`, at a case count identical in both configurations, and `scripts/measure-ltx2-prompt-adaln.py` is a manual tool no gate invokes. Listed under `## Owed` in [`ltx25-prompt-adaln.md`](specs/ltx25-prompt-adaln.md) | bug |
 | [#900](https://github.com/mudler/vllm.cpp/issues/900) | — | LTX-2.5 prompt-AdaLN is REACHED from `vllm_video_generate` but no test ENTERS through that entry point and observes it: deleting the term's only consumer (`ltx2_dit.cpp:140`) takes `test_ltx2` red at 3 of 43 cases while `test_ltx2_video` stays GREEN at 37 of 37, so the ABI-level gate measures that the pipeline runs rather than that this capability is in it. The second half of `AGENTS.md` `## Nothing lands dead`, whose rule post-dates the row (#886). Listed under `## Owed` in [`ltx25-prompt-adaln.md`](specs/ltx25-prompt-adaln.md) | bug |
 | [#891](https://github.com/mudler/vllm.cpp/issues/891) | `MODEL-MM-qwen3-5-qwen3-5-moe-for-conditional-generation` | MoE vision tower (M2/M3): image and video for `Qwen3_5MoeForConditionalGeneration` — the dense arm has it, the MoE arm does not | feature |

docs/USAGE.md

@@ -2032,11 +2032,26 @@ only when it reproduces the identical weight contract the shapes describe, and
 supplying one for a checkpoint that already declares its own is refused rather
 than ordered.
 
+`vllm_video_model_params.device` is `0` for the CPU and `1` for **the
+accelerator this build resolves** — not for CUDA. The value is unchanged and it
+is CUDA on a CUDA build, but it is read through the platform seam rather than as
+an enum value, so the same `1` selects Metal, Vulkan or Tenstorrent on a build
+that registers one of those, and is refused by name on a build that registers
+none. The C ABI's text-generation `vllm_model_params.device` is a separate,
+later selector with its own `0 = auto / 1 = cpu / 2 = cuda` numbering.
+
 The LTX-2.5 arm runs on the CPU in f32 and on CUDA in bf16. `device = 0` takes
 the f32 parity forward; `device = 1` stages the DiT to the GPU one tensor at a
 time and runs the device-resident forward, so a CUDA handle means a CUDA forward.
-On a build with no CUDA backend, `device = 1` is refused by name rather than
-served the CPU forward behind a CUDA handle. `encoder_path` loads the Gemma-4
+On a build with no accelerator backend, `device = 1` is refused by name rather
+than served the CPU forward behind an accelerator handle. It is also refused when the build's
+accelerator is a PARTIAL backend that declines this architecture — Metal and
+Tenstorrent each register the kernels for a named short list of models, and a
+backend that has not registered this one now says so by name instead of binding
+a queue and failing later inside a kernel. The same three questions decide
+`minimax-h3`'s `device = 1`, which resolves through the platform seam rather
+than reading the ABI selector as an enum value, so on a CPU-only build it throws
+instead of naming CUDA. `encoder_path` loads the Gemma-4
 text tower, and the request's own `prompt` then conditions the render; the tower
 itself runs on the CPU in f32 whichever device the DiT is on. Without one,
 conditioning comes from the two prompt-embeds files, which must agree on their

include/vllm.h

@@ -439,8 +439,9 @@ typedef struct vllm_model_params {
    *        silently replaced by another (mirror of vLLM assigning an explicit
    *        device verbatim, device.py:61-66).
    * 0 must stay auto so a zero-initialized struct preserves pre-v14 behaviour;
-   * the cpu-before-cuda value order follows the v12 precedent
-   * (vllm_video_model_params.device: 0 cpu, 1 cuda) shifted by the auto slot.
+   * the cpu-before-accelerator value order follows the v12 precedent
+   * (vllm_video_model_params.device: 0 cpu, 1 the resolved accelerator)
+   * shifted by the auto slot.
    * Any other value fails vllm_engine_load with VLLM_ERR_INVALID_ARGUMENT. */
   int32_t device;
   /* ── KV-pool sizing (ABI v16) ──────────────────────────────────────────────
@@ -874,7 +875,13 @@ typedef struct vllm_video_model_params {
    * are byte-structurally identical, so it must be DECLARED; NULL/empty makes
    * every generate refuse with the guidance (the #77 guard). */
   const char* partition;
-  int32_t device;       /* 0 cpu, 1 cuda */
+  /* 0 is the CPU; 1 is THE ACCELERATOR THIS BUILD RESOLVES, through the
+   * platform seam (CurrentPlatform + TryGetBackend +
+   * supports_model_architecture), never the enum value 1. It is therefore CUDA
+   * on a CUDA build and refused BY NAME on a build with no accelerator backend,
+   * or one whose partial backend declines this architecture (#659, #660). The
+   * ABI value is unchanged; what it means was never "cuda". */
+  int32_t device;
   int32_t dequant_bf16; /* 0 keep-quant, 1 dequant/stream bf16 */
   int32_t fp4_resident; /* NVFP4+cuda: keep FP4 packed, Marlin W4A16 GEMM */
   /* ── v18 additions (the generalized seam) ─────────────────────────────────

include/vllm/config/device.h

@@ -15,8 +15,9 @@
 //   - The integer values are the C-ABI wire contract
 //     (vllm_model_params.device, ABI v14): 0 MUST be auto so a zero-initialized
 //     struct preserves the pre-v14 accelerator-first behaviour byte for byte;
-//     cpu-before-cuda then follows the shipped v12 precedent
-//     (vllm_video_model_params.device: 0 cpu, 1 cuda) shifted by the auto slot.
+//     cpu-before-accelerator then follows the shipped v12 precedent
+//     (vllm_video_model_params.device: 0 cpu, 1 the accelerator that build
+//     resolves) shifted by the auto slot.
 #ifndef VLLM_CONFIG_DEVICE_H_
 #define VLLM_CONFIG_DEVICE_H_
 

include/vllm/multimodal/minimax_h3_video.h

@@ -58,8 +58,14 @@ namespace vllm::multimodal {
 inline constexpr char kMiniMaxH3VideoFamily[] = "minimax-h3";
 
 // Map the stable public video ABI device selector onto the runtime's generic
-// backend key. The ABI remains 0=CPU / 1=CUDA; callers below this seam dispatch
-// only through the returned DeviceType.
+// backend key. The ABI is unchanged — 0 is the CPU, 1 is the accelerator, and
+// anything else throws — but 1 is RESOLVED through the platform seam
+// (`CurrentPlatform().device_type()` + `TryGetBackend` +
+// `supports_model_architecture`), never cast from the integer. It therefore
+// THROWS on a build with no accelerator backend, or one whose partial backend
+// declines this architecture, instead of naming a device that build cannot run
+// (#659, #660). Callers below this seam dispatch only through the returned
+// DeviceType.
 vt::DeviceType MiniMaxH3VideoDeviceType(int32_t device);
 
 // ── Load-time parameters (the checkpoint set; the C ABI mirror is
@@ -79,7 +85,9 @@ struct MiniMaxH3VideoModelParams {
   // declared-but-unknown and the #77 guard refuses every full render
   // (MiniMaxH3PartitionFromFlag / MiniMaxH3CheckTaskPartition).
   std::string partition;
-  int32_t device = 0;        // 0 cpu, 1 cuda
+  // 0 cpu, 1 the accelerator this build resolves — see MiniMaxH3VideoDeviceType
+  // above; never `static_cast<vt::DeviceType>(device)` (#660).
+  int32_t device = 0;
   int32_t dequant_bf16 = 0;  // 0 keep-quant, 1 dequant/stream bf16
   // NVFP4 + cuda only: keep the packed FP4 resident and route the quantized
   // projections through the Marlin W4A16 GEMM (the pre-fold --fp4-resident).

include/vllm/multimodal/video_engine.h

@@ -59,7 +59,10 @@ struct VideoModelParams {
   // value that is not registered is refused (it is never treated as a hint).
   std::string family;
 
-  int32_t device = 0;        // 0 cpu, 1 cuda
+  // 0 is the CPU; 1 is the accelerator this build RESOLVES through the platform
+  // seam, not the enum value 1 (#659, #660). Refused by name when this build has
+  // no accelerator backend, or its partial backend declines the family.
+  int32_t device = 0;
   int32_t dequant_bf16 = 0;  // 0 keep-quant, 1 dequant/stream bf16
   int32_t fp4_resident = 0;  // keep packed FP4 resident + quantized GEMM
   int64_t encoder_max_layers = 0;  // 0 => all layers